The invention relates to the field of observation and imaging from a spacecraft. More particularly, in one aspect, the invention provides a camera device on board such a spacecraft. For example, the camera device may include a charge-coupled device (CCD) sensor matrix. Such CCD sensors are also referred to below as xe2x80x9cphotosensitive charge transfer elementsxe2x80x9d.
More particularly, the invention relates to such camera devices including means for compensating smearing due to movements of the spacecraft relative to the object to be observed and/or whose image is to be conserved.
For example, the invention can be used for imaging the ground from a satellite. Under such circumstances, in order to obtain good resolution, it is necessary to ensure that sufficient energy reaches each sensor during the time the satellite travels a distance corresponding to the desired resolution on the ground.
Known sensor matrices that are available on the market present radiometric performance requiring an exposure time of about 500 microseconds (xcexcs) for a signal-to-noise ratio of 40 decibels (dB). For a satellite traveling at 8 kilometers per second (km/s) with ground resolution Rs equal to 1 meter (m), then exposure time cannot exceed xe2x85x9th of a millisecond (ms), i.e. 120 microseconds (xcexcs). This exposure time should even be shorter if it is desired maintain good image sharpness.
The quantity of light picked up by each sensor during such an exposure time is therefore insufficient.
To obtain the necessary quantities of light, telescopes have been used with apertures that are 3 to 4 times that which is required by diffraction conditions, so as to pick 10 times more energy. However such telescopes are heavy and bulky (with their weight increasing with the square of their aperture).
Another way of obtaining a sufficient quantity of light is to lengthen exposure time and to compensate smearing during the exposure time by synchronizing displacement of the camera device relative to the satellite as a function of the motion of the satellite along its trajectory. This compensation of smearing is sometimes achieved by modifying light paths, by moving a mirror, a lens, etc.
Document U.S. Pat. No. 5,460,341 describes a camera device for use on board a satellite or other spacecraft, the camera device including a smearing compensation system to correct displacements along an optical axis, focusing displacements of optical parts, the position of the camera device itself relative to the craft on which it is fixed, etc. That compensation device includes linear actuators for adjusting the position of the camera device relative to the spacecraft by controlled displacement in several degrees of freedom. The actuator comprises a motor having a moving coil or a moving magnet. Those linear actuators are of the contactless type, i.e. there is no friction contact between the moving parts relative to one another. Unfortunately, such devices become unusable when it is desired to obtain very high resolution in the compensation movement. Under such circumstances, extremely precise displacements and very short response times are required, but cannot be obtained with those prior art camera devices.
An object of the invention is to provide camera devices with a displacement mechanism that is more precise and that has a response time that is very short.
According to the invention, this object is achieved by a camera device for mounting on board a spacecraft, the camera device having at least one observation sensor and displacement means (4, 5, 6, 7, 8, 9) for controlled displacement of said device relative to the spacecraft with at least one degree of freedom, the device being characterized in that said displacement means comprise at least one transducer of an active material suitable for deforming dynamically under the effect of a variable electric and/or magnetic field, together with driver means suitable for controlling the transducer so as to displace the observation sensor in compliance with a variable control relationship adapted to compensating movements induced by the flight dynamics of the satellite.
This type of material makes it possible to obtain displacement of the entire camera device relative to the spacecraft with positioning that is rigid, with high precision, and practically without any time delay and without using mechanical structures proper.
The use of an active material suitable for deforming is under the effect of an electric field and/or a magnetic field, typically a piezoelectric material or a magnetostrictive material, can be envisaged only for displacements that are very small. However, the optical resolutions that were accessible with prior art devices were such as to require compensation displacements of an amplitude that was too great for it to have been possible to envisage using such materials.
The camera device of the invention may present the following advantageous but optional characteristics taken separately or in combination:
the displacement means comprise a plurality of transducers, each transducer producing displacement of the sensor relative to the spacecraft that is orthogonal relative to the displacement produced by each other transducer;
servo-control means are also provided for each transducer, suitable for compensating disturbing movements affecting the position of each sensor relative to an object to be observed;
the active material is mounted directly on a matrix of observation sensors to form an integrated structure; and
each observation sensor is a photosensitive charge transfer element.
In another aspect, the invention provides a device for causing a camera device to be displaced in controlled manner in at least one degree of freedom relative to the spacecraft. It further comprises displacement means comprising a transducer of an active material suitable for deforming under the effect of an electric field and/or a magnetic field. Such a device makes it possible in particular to compensate smearing associated with the movements of a spacecraft relative to an object that is remote from the craft, and in particular movements corresponding to displacements along a trajectory.
Such a device, coupled with a control system that generates a voluntary displacement relationship depending on the behavior of the platform on which the camera device is mounted, together with a camera device of the desired type, makes it possible at negligible extra cost and weight to provide a function serving to obtain maximum resolution and scanning width for an optical system carried by a spacecraft, without giving rise to energy losses.
In yet another aspect, the invention provides a spacecraft including a camera device as specified above.
In yet another aspect, the invention provides a method of imaging in space, the method including an operation consisting in displacing a camera device relative to a spacecraft, the method being characterized in that this operation is implemented by applying an electric field and/or a magnetic field to deform an active material interposed between the camera device and the spacecraft, and in that said transducer is driven in such a manner as to displace the observation sensor in compliance with a variable control relationship adapted to compensating the movements induced by the flight dynamics of the satellite.
In advantageous but optional manner, the method of the invention presents the following characteristics taken independently or separately:
it includes an operation consisting in compensating the smearing associated with the movements of a spacecraft relative to an object to be observed that is remote from said spacecraft, and in particular the movements corresponding to the displacement of said spacecraft along a trajectory;
it includes an operation consisting in using the active material to cause a matrix of observation sensors extending mainly in a plane to tilt relative to the tangent of the trajectory of the spacecraft;
it includes an operation consisting in taking a first image of the object to be observed from a first position on the trajectory, and then a second image of said object from a position downstream from said first position, in order to form a stereoscopic image of the object;
it includes an operation consisting in displacing the camera device laterally relative to the trajectory of the spacecraft to form a stereoscopic image of the object;
it includes an operation consisting in causing the camera device to turn about an axis perpendicular to the trajectory in order to form a stereoscopic image of the object to be observed; and
it includes an operation consisting in taking a plurality of images corresponding to the camera device occupying positions that are juxtaposed beside one another by displacing the camera device between taking two images, the device being displaced laterally relative to the trajectory of the spacecraft so as to form, after processing all of the images, an image of the object to be observed that corresponds to a field that is wider than that which would be obtained using only one of said images.